The invention relates to the field of intravascular medical devices, and more particularly to a balloon for a catheter.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient's coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient's coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion.
Catheter balloons are typically manufactured independently of the catheter shaft and then secured to the catheter shaft with an adhesive or other bonding method. In standard balloon manufacture, a polymer tube is blown biaxially under the action of axial tension, internal pressure, and heat within a mold. The polymer tube may either be simultaneously stretched in the radial and axial directions, or sequentially, by first stretching axially and then radially. The starting dimensions of the polymer tube and the finished dimensions of the blow-molded balloon within the mold are a measure of the degree to which the polymeric material has been stretched and oriented during balloon blowing, and affect important characteristics of the finished balloon such as rupture pressure and compliance. The blow-up-ratio (BUR), namely, the ratio of the outer diameter of the blown balloon (i.e., the mold inner diameter) to the inner diameter of the polymer tube, is a measure of those dimensions. Beyond a critical BUR for a given polymer, the balloon blowing process becomes unstable and the polymer tubing often ruptures or tears before a balloon is fully formed.
In the standard blow molding process, an initiated bubble rapidly grows in diameter until it is constrained by the mold wall. The hoop stress in the wall of the tubing, as it grows into a balloon, may be approximated by the expression:σh=(P·R)/δwhere P is the inflation pressure, R is the mean radius of the polymeric tube at any time during the inflation and δ, delta, is the wall thickness of the tubing. For a balloon to be initiated from the tubing, the inflation pressure should be such that the wall hoop stress exceeds the material resistance (typically the yield stress) to stretching at the blowing temperature. Once a balloon is initiated from the tubing, it grows rapidly in size until it touches the mold wall. As the balloon grows, the radius increases and the balloon wall thickness decreases. This results in a rapid increase in the wall hoop stress during constant pressure blowing. If the wall hoop stress of the growing balloon exceeds the ultimate hoop strength of the material, rupture will occur. As a result, there is a limit to the BUR (i.e., a maximum attainable BUR) of a polymeric material forming the balloon layer(s).
In the design of catheter balloons, balloon characteristics such as strength, flexibility and compliance must be tailored to provide optimal performance for a particular application. Angioplasty and stent delivery balloons preferably have high strength for inflation at relatively high pressure, and high flexibility and softness for improved ability to track the tortuous anatomy and cross lesions. The balloon compliance, which depends on factors such as the nature of the balloon material, the balloon wall thickness, and processing conditions, is chosen so that the balloon will have a desired amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon the application of tensile force. Noncompliant balloons, for example balloons made from materials such as PET, exhibit relatively little stretching during inflation, and therefore provide controlled radial growth in response to an increase in inflation pressure within the working pressure range. However, noncompliant balloons generally have relatively low flexibility and softness, so that it has been difficult to provide a low compliant balloon with high flexibility and softness for enhanced catheter trackability. A balance is typically struck between the competing considerations of softness/flexibility and noncompliance, which, as a result, has limited the degree to which the compliance of catheter balloons can be further lowered.
Therefore, what has been needed is a catheter balloon with very low compliance, yet with excellent ability to track within the patient's vasculature and cross lesions therein. The present invention satisfies these and other needs.